Superimposed coded signaling systems



Jan. 24, 1956 w. 1.. MQNABB 2,732,490

SUPERIMPOSED CODED SIGNALING SYSTEMS Filed Feb. 27 1951 INVENTOR.

LMQ Vqzbb. BY 2 Hi5 ATTORNEY United States Patent SUPERIMPOSED CODEDSIGNALING SYSTEMS William L. McNabb, Turtle Creek, Pa., assignor toWestinghouse Air Brake Company, a corporation of PennsylvaniaApplication February 27, 1951, Serial No. 212,884

11 Claims. (Cl. 246-34) My invention relates to coded signaling systems,and particularly to an improved coded signaling system which may beapplied to a conventional steady energy signaling system. Moreparticularly, my invention relates to the application of a codedsignaling system to a conventional direct current railway track circuit.

A conventional railway track circuit usually includes a source of trackcircuit energy, such as a battery, connected across the track rails atone end of a section or stretch of railway track, and a direct currentrelay connected across the rails at the other end of the stretch. Energyfrom the track battery flows over the rails and through the winding ofthe relay, sothat the relay is energized and its contacts are picked upwhen the section is unoccupied. When a train occupies the section, thewheels and axles of the train shunt the supply of energy from thewinding of the relay, so that its contacts are released. Accordingly,the usual direct current railway track circuit provides only for thedetection of the occupancy of a track section by a train. In manyinstances, it is desirable to transmit further information including thetrack occupancy conditions along the rails of the track, in order toindicate various traffic conditions which may exist at some point inadvance. It has previously been proposed to provide such additionalchannels of communication by the use of the well-known polarized trackcircuits, and by the use of the coded type of track circuits, in whichthe supply of energy to the track section from the track battery isrecurrently interrupted, and the code'following operation of the trackrelay is detected by suitable means.

However, it may at times be desirable to provide a conventional directcurrent track circuit, arranged so that additional information may betransmitted over the rails of the track section involved, withoutinterfering with the normal operation of the track circuit.

Accordingly, it is an object of my invention to provide a superimposedcoded signaling system which may be applied to conventional directcurrent track circuits, to provide an additional channel ofcommunication over the track rails.

Another object of my invention is to provide a system of the typedescribed in which the track rails form an indication channel which maybe unidirectional or bidirectional, that is, information may betransmitted in one direction or in both directions, without interferingwith the normal operation of the track circuit.

A further object of my invention is to provide a system of the typedescribed in which the usual track battery employed for supplying energyto the track circuit may be utilized to provide the energy for theoperation of the superimposed coded signal arrangement.

Other objects of my invention and features of novelty thereof will beapparent from the following description taken in connection with theaccompanying drawings.

In practicing my invention, I provide,-in addition to the usual trackbattery connected across the track rails at one end of the track stretchinvolved and the track 2,732,4ild iatented Jan. 24, 1956 ICC relayconnected across the section rails at the other end of the stretch,means located at one end of the stretch, for recurrently changing thevalue of current flow in the track circuit, and means at the other endof the stretch responsive to such changes in the current flow, togetherwith appropriate means for detecting the recurrent operation of theapparatus involved. In one form of my invention, 1 provide meansincluding a resistance and a recurrently operated coding contact forrecurrently changing the value of energy at the end of the track sectionwhere the conventional track relay is located. At the battery end of thetrack section, I provide a transformer having one winding connected inseries with the track battery across the section rails, and havinganother winding connected to the winding of a code following relay. Therecurrent operation of the coding contact at the track relay end of thesection causes the value of energy supplied from the track battery to berecurrently changed. This recurrent change in the energy suppliedthrough the one winding of the transformer causes impulses of energy tobe induced in the other winding, which impulses are supplied to thewinding of the code following relay, causing it to operate its contactsin synchronism with the operation of the coding contact at the oppositeend of the section. Appropriate code detecting means are provided fordetecting the recurrent operation of the contacts of the codefollowing'relay. p

In a second embodiment of my invention, I provide a transformer at eachend of thetrack circuit, one having one of its windings connected inseries with the track relay across the section rails, and the otherhaving a winding connected in series with the track battery across thesection rails. The transformers are each provided with a circuitincluding a coding contact, a battery, and a code following track relay,and the apparatus is arranged and proportioned so that when the codingcontact is recurrently operated, energy from the battery flows throughone winding of the transformer, in such manner as to induce an impulseof energy in the other winding of the transformer connected with thesection rails. This impulse upon reception at the opposite end of thetrack circuit causes a like impulse to be induced in the winding of thetransformer connecetd at that end of the section, to thereby operate thecode following relay associated with the other transformer. Thisarrangement is suitable for transmitting the coded information in eitherdirection through the conventional track circuit.

I shall describe two preferred embodiments of my invention and shallthen point out the novel features thereof in claims.

In the accompanying drawings,

Fig. 1 is a diagrammatic view of a section of railway track equippedwith apparatus embodying my invention, for transmitting codedinformation over a conventional track circuit in one direction only.

Fig. 2 is a diagrammatic view of a section of railway track equippedwith apparatus embodying my invention for transmitting coded informationin either direction over a conventional track circuit.

Similar reference characters refer to similar parts in each of the twoviews.

Referring to the drawings, Fig. 1 shows a section of railway track Tseparated from adjoining sections by the usual insulated joints 3 in thetrack rails 1 and 2. At the right-hand end of the section, there isprovided the usual track battery TB, a limiting resistor R2, an impulsetransformer TT, 2. code following polar stick relay CTR, a decodingtransformer DT, and a code detecting relay CH. At the left-hand end ofthe section, there is provided the usual direct current neutral trackrelay TR, a pair of resistors R1 and R3, a code transmitting relay CTPR,governed by a control switch S1 and a continuously-operating codingdevice CT. At each end of the section, energy for the Operation of theapparatus other than the track circuit equipmen may be furnished by asuitable source of low voltage direct current energy, such as thebatteries LB, having their positive terminals designated by thereference character B and their negative terminals designated by thereference character N.

The apparatus is shown in its normal condition, with the section Tunoccupied, and with the code transmitting relay CTPR deenergized. Thecode transmitter or ccdi device CT has its operating windingcontinuously connected across the terminals of the direct currentsource, and is of the type having at least one contact which isrecurrently operated between its open and closed position at apredetermined rate, for example, 180 times per minute. Accordingly, itwill be seen that contact a of coding device CT is recurrently openedand closed 180 times per minute, but no energy is supplied over thefront Contact a of coding device CT to the relay CT PR, since the switchS1 is open.

At this time also, the neutral track relay TR is energized by currentflowing from the positive terminal of battery TB, through the winding 7of transformer TT. over section rail 2, through resistor R1, through thewinding of relay TR, and over section rail 1 and through resistor R2 tothe negative terminal of battery TB. The components are proportioned andarranged so that the value of current which flows through the winding ofrelay TR at this time is sufficient to pick up the contacts of therelay. Accordingly, at this time the front contact a of relay TR will beclosed in a control circuit, which may be used, for example, to controla signal governing the movement of trains into section T, or any othertype of circuit which must respond to the occupancy of section T.

It will now be assumed that the switch S1 is closed, so that relay CTRPis recurrently energized by energy supplied from the low voltage sourceover the front contact a of the coding device CT. Accordingly, thecontacts a and b of relay CTPR are recurrently picked up and released,in response to the operation of the coding contact a of coding deviceCT. When front contact I) of relay CTPR is closed, it establishes ashunt around the resistor R1 in the circuit for supplying energy to thewinding of the neutral track relay TR, and when front contact a of relayCTPR is closed it provides a shunt circuit across the resistor R1 andthe winding of relay TR, through the resistor R3 and front contact b.Accordingly, it will be seen that the resistor R3. is elfectivelyconnected in multiple with the winding on track relay TR across thesection rails 1 and 2 at this time. The 'value of resistor R3 is chosenso that the current which flows through the winding of track relay TR atthis time will not be reduced below the point where the contacts of therelay will release, since, as is well-known in the 'art, the releasevalue of current for the track relay TR is substantially less than thepick up value. Accordingly, while the contacts a and b of codetransmitting relay CTPR are recurrently opened and closed, the contactsof track relay TR will remain in their picked up position, since thecurrent through the winding is not reduced below the drop away for theparticular relay employed.

However, the change in current produced by the alternate shunting of theresistor R3 across the winding of relay TR and the short circuiting ofresistor R1 produces a change in the value of current which flows fromthe track battery TB. The change of current flowing in the winding 7 oftransformer TT causes energy to be induced in the second winding 9 oftransformer TT, the polarity of the energy being alternately reversed inaccordance with the increase or the decrease in the value of currentflowing through the winding 7. Thus, it will -be 'seen that therecurrent changes in the current fi'owing through the Winding 7 oftransformer TT as the res'ult of the shunting operation of the contactsof relay CT PR at the opposite end of the track circuit will causeimpulses of energy of alternate opposite polarity to be induced in thesecond winding 9 of the transformer TT, which impulses are supplied tothe winding of the polar stick code following relay CTR. Relay CTR is ofthe type in which the contacts remain in the position to whicn they werelast operated, until the relay is energized with energy of oppositepolarity, whereupon the contacts will move to corresponding position,and remain there when the relay is deenergized. At this time, therefore,it will be seen that the winding of relay CTR is supplied with pulses ofenergy of alternate opposite polarity from the winding 9 of transformerTT, and as a result, the contacts of relay CTR are recurrently operatedbetween their normal or left-hand position, and their reversed orright-hand position. The code following opcration of relay CTR may bedetected by any of the means well-known in the art, and as here shown adecoding transformer and a slow release, slow pick up relay are employedto detect the code following operation of the relay contacts. The twohalves of the primary winding 13 of the decoding transformer DT arealternately supplied with energy from the direct current source over thecontact a of relay CTR, and the energy induced in the secondary winding15 is mechanically rectified by the action of contact b of relay CTR andis supplied to the winding of the code detecting relay CH. Relay CH isslightly slow in releasing its contacts, so that it remains picked upduring the intervals in which no energy is supplied to its winding, thatis, while the contacts of relay CTR are transferring from one positionto the other. Such a code detecting arrangement is more fully describedin Letters Patent of the United States No. 2,237,788, issued to Frank H.Nicholson et al., and a detailed explanation of the operation of theequipment is deemed unnecessary. The contacts of relay CH may be used tocontrol any suitable circuits, such as for example, the control ofwayside signals for the section T, the control of approach indicationsat an interlocking plant or other control circuits.

If it is now assumed that a train enters and occupies section T, it willbe seen that the wheels and axles of the train will shunt the supply ofenergy from the winding of the neutral track relay TR, so that relay TRwill release its contacts and they will remain released as long as thetrack section T is occupied. Moreover, the train shunt presents arelatively low and substantially constant resistance to the fioW ofenergy from the battery TB, so that the rate of change of currentflowing in the winding 7 of transformer TT, if any, during the time thatthe section T is occupied, will be insufficient to cause the operationof relay CTR. Accordingly, the relay CTR will stop operating itscontacts, and after a short time interval, the code detecting relay CHwill release its contacts, thereby opening or closing any circuitsgoverned by this relay.

When the train vacates section T, the supply of energy from battery TBover the section rails 1 and 2 to the winding of track relay TR is againresumed, so that relay TR picks up its contact a at least during thefirst time period in which contacts a and b of relay CTPR are released,and thereafter remains picked up, thereby opening or closing thecircuits governed by this contact. Additionally, the resistor R3 isrecurrently shunted across the winding of relay TR, so that the value ofenergy flowing in the transformer TT is changed, thereby supplyingimpulses of induced energy to the winding of the code following relayCTR, so that its contacts again recurrently operate between their normalor reversed position. With the contacts a and b of relay CTR recurrentlyoperating, energy is supplied over the contact to the primary winding 13of the decoding transformer DT, and the energy induced in the secondarywinding 15 of the transformer DT is rectified by the contact 1') ofrelay CTR and supplied to the winding of the code detecting relay CH, sothat the contact of relay CH picks up. The circuits controlled overcontact a of the code detecting relay CH '5 are accordingly opened orclosed, to indicate the absence of a train from the section T, and thetransmission of additional communication as a result of the switch S1being closed.

It will be noted that if the relay CTPR is deenergized by the opening ofthe switch 81- during the time a train occupies the section T, thechange in the flow of energy from the battery TB through the primarywinding 7 of transformer TT when the train vacates the section may besufficient to induce an impulse of energy in the secondary winding ofthe transformer, which may operate the contacts of relay CTR from oneposition or the other, according as the contacts were last energized intheir normal or reverse position. This so-called single stroke operationof the contact of the code following relay CTR will not result in thepicking up of the code detecting relay CH, since this relay isconstructed and arranged not only to be slow in releasing its contacts,but additionally to be slow in picking up its contacts, so that energysupplied to its winding as a result of the single operation of thecontacts of relay CTR is insufiicient to cause the relay to pick up itscontact a. Accordingly, a single change of the rate of current flow tothe transformer TT will not cause erroneous operation of the equipment.

It will be seen from the foregoing, that a track circuit arranged inaccordance with my invention provides means for supplying additionalcommunication or information over a conventional track circuit, whichrequires a minimum amount of apparatus, is reliable, and utilizes theconventional track battery as a source of energy for providing the extracommunication channel over the track. It will also be seen that thisadditional channel provided by the coding of track circuit energy isresponsive to the presence of the train in the section. Accordingly, itwill be obvious to those skilled in the art that the contacts of theconventional track relay such as the relay TR may be used to govern theusual wayside signal equipment, while the additional channel furnishedby the coding apparatus may be used for governing circuits such ashighway crossing approch control circuits, approach indication circuits,and other circuit arrangements of this type. Moreover, it will be seenthat the arrangeinent is not limited to use with track circuits, but maybe used with circuits in which the conductors are line wires, insteadofthe rails of a track section. Accordingly, a neutral type line circuitmay be substituted for the track rails 1 and 2, and by providingsuitable shunting or circuit opening means for the line circuit, thesame eifects will be secured as described in connection with Fig. 1.

Referring now to Fig. 2, there is shown a section of railway track Thaving track rails 1 and 2, and'insulated from the adjacent sections byinsulated joints 3. This track section is provided with a conventionaldirect current track circuit including the track battery TB and thetrack relay TR connected across the rails at the opposite ends of thetrack section. Additionally, apparatus is provided for superimposingimpulses upon the direct current track circuit energy, so that anadditional channel of communication is afforded, which may be operatedin either direction, as will be subsequently described. This additionalapparatus comprises a transformer such as the transformers L'IT and RTTconnected in series with the track leads at each end of the section, acode transmitting relay such as the relays LCTPR and RCTPR, which areeach governed by a code transmitter and a control switch, a codefollowing relay such as relays LCTR and RCTR, and a source of codeenergy such as the batteries LCTB and RCTB, and code detecting equipmentfor detecting the recurrent operation of the code following relays, suchas the transformers RDT and LDT, with their-associated code detectingrelays RCH and LCH. As in Fig. l, the energy for the operation of theapparatus other than that associated directly with the track circuit isfurnished at each end of the track circuit by a suitable source of lowvoltage direct current energy, such as the batteries LB, each having apositive terminal designated by the reference character B and a negativeterminal designated by the reference character N, respectively.

The arrangement of apparatus shown in Fig. 2 may be more clearlyunderstood by describing the operation of the apparatus under variousconditions. It will first be assumed that the apparatus is in its normalcondition, as shown, with no train occupying the section T. At thistime, the neutral track relay TR is supplied with energy from the trackbattery TB, by a circuit which may be traced from the positive terminalof the battery TB, through the winding 19 of transformer LTT, oversection rail 2, through the winding 21 of transformer RTT, through thewinding of the track relay TR, over section rail 1, and through theresistor R2 to the negative terminal of battery TB. Accordingly, therelay TR will be energized and its contact a will be picked up tothereby control any circuits which may be governed thereby.

The windings of the code transmitters LCT and RCT are continuouslyconnected to the source of low voltage energy, and accordingly thecontacts are recurrently opened and closed at a predetermined rate, forexample, 180 times per minute. However, with the control switches LS andRS open, the code transmitting relays LCTPR and RCTPR will bedeencrgized.

It will now be assumed that it is desired to transmit coded energy overthe track section T from the left-hand end of the section to theright-hand end of the section. Accordingly, the contact of switch LS isclosed, so that coded energy is supplied by an obvious circuit to thewinding of relay LCTPR. Accordingly, times per minute the contact a ofrelay LCTPR will operate between its front and back contacts. When thecontact a of relay LCTPR picks up, it establishes a circuit forsupplying energy from the battery LCTB to the winding 23 of transformerRTT. When the contact a of relay. LCTPR releases, it disconnects thebattery LCTB from the winding 23 of transformer RT and connects thewinding of the code following relay RCTR to the winding 23 of thetransformer. The supply of impulses of energy from the battery LCTB tothe winding 23 of transformer RTT causes impulses of energy to beinduced in the winding 21 of the transformer RTT. The condenser TQ,connected across the winding of relay TR, aifords a low impedance pathfor these impulses to be supplied across the rails 1 and 2 of section T.These impulses are accordingly supplied over the rails 1 and 2 ofsection T to the right-hand end of the section, and flow through thewinding 19 of transformer LTT, the battery TB and the resistor R2. Theparts are proportioned and arranged so that the pulses of energy thusinduced in the winding 25 of transformer LTT are suificient to causeoperation of relay LCTR, which has its winding connected to the winding25 of transformer LTT over the back contact a of relay RCTPR.

The impulses of energy supplied from the winding 25 of the transformerLTT, over back contact a of relay RCTPR to the winding of the codefollowing. relay LCTR are of relative opposite polarity, and as aresult, the contacts of relay LCTR are recurrently operated betweentheir normal and reverse positions, in step with the supply of energyfrom the battery LCTB over the front contact a of relay LCTPR to thewinding 23 of transformer RTT. The recurrent operation of the contacts aand b of relay LCTR is detected by means of the conventional codedetecting relay LCH, in conjunction with the decoding transformer LDT.When the contacts of relay LCTR .areoperating, impulses ofunidirectional energy are supplied from the transformer LDT to thewinding of relay LCH, so that the contact a of relay LCH is picked up tothereby open or close the control circuits governed by this relay.

From the foregoing, it will be seen that at this time with the tracksection T unoccupied, the neutral track relay TR is energized by theenergy supplied over the track rails from the conventional track relayTB, and there is superimposed upon this energy impulses of low frequencyenergy supplied from the transformer RTT, which pulses are supplied tothe transformer LTT to operate the code following relay LCTR.

If it is desired to transmit information over the section T from theright-hand end to the left-hand end of the section, and assuming thatthe apparatus is in its normal condition as shown in the drawing, theswitch RS may be closed at the right-hand end of the section, to therebyrecurrently energize the code transmitting relay RCTPR, so that itrecurrently connects the battery RCTB to the winding of the transformerLTT. The impulses of energy induced in the winding 19 of transformer LTTare superimposed upon the direct current energy supplied from batteryTB, and flow through the winding 21 of transformer RTT at the left-handend of the section, and through the condenser TQ which by-passes thewinding of the conventional track relay TR. The impulses thereby inducedin the winding 23 of transformer RTT are supplied to the winding ofrelay RCTR over back contact a of the code transmitting relay LCTPR,which relay is deenergized at this time by the opening of the switch LS,so that relay RCTR operates its contacts in response to the recurrentsupply of energy to the transformer LTT at the other end of the section.As in the case of relay LCTR, the code following operation of relay RCTRis detected by the conventional arrangement of the code detectingtransformer RDT and code detecting relay RCH, so that when relay RCTR isfollowing code, the relay RCH will be energized and its contact a willopen or close the control circuits governed by this relay.

From the foregoing, it will be seen that the track circuit arrangementshown in Fig. 2 and embodying my invention is arranged so that anadditional communication channel is provided over a conventional directcurrent track circuit, in which the information may be transmitted fromeither end of the section to the other end of the section. As pointedout in connection with Fig. l, the conventional track relay TR maygovern wayside signals for controlling the movements of trains into thesection T, while the code apparatus may control auxiliary circuits, suchas highway crossing approach control circuits, approach lightingcircuits, etc.

Additionally, as was pointed out in connection with Fig. l, the entranceof a train into section T will cause the track circuit to be shunted, sothat not only is the conventional track relay TR released, but thesupply of code impulses, if they are being transmitted at this time, iscut off, so that the code detecting relay would at the appropriate endof the section become deenergized. When the train enters or leaves thetrack section, particularly at the right-hand end where the trackbattery TB is located, it will be seen that a change in the value ofcurrent flowing in the winding 19 of transformer LTT will take place.However, the impulse induced in the winding 25 of transformer LTT bythis action will only cause the code following relay to move itscontacts from one position to the other, and the code detectingapparatus is arranged and constructed so that the code detecting relay,such as relay LCH, which is not only slow in releasing its contacts butalso slow in picking them up, will not pick up its contacts from theso-called single stroke operation.

From the .above description of two forms of railway signaling systemsembodying my invention, it will be apparent that I have provided asimple and reliable means of superimposing an additional communicationchannel upon a steady energy direct current railway track circuit, andalthough I have .herein shown and described only two forms ofsuperimposed coded signaling systems embodying my invention, it is to beunderstood that various changes and modifications may be made thereinwithin the scope of the appended claims without departing from thespirit and scope of my invention.

Having thus described my invention, what I claim is:

l. in combination, a pair of conductors, a source of direct currentenergy connected across said conductors at one end thereof and anelectroresponsive device connected across said conductors at the otherend thereof, said electro-responsive device being responsive to a predetermined minimum value of energy supplied thereto over saidconductors, means located at one end of said pair of conductors forrecurrently changing the value of energy supplied from said source tosaid electro-responsive device between a first value which is greaterthan said predetermined minimum value and a second value which isgreater than said first value, and means located at the other end ofsaid pair of conductors responsive to changes of energy supplied fromsaid source to said conductors, when said changes are recurrent and areat least as great as the difference between said first and said secondvalues.

2. In combination, a pair of conductors, a source of direct currentenergy connected across said conductors at one end thereof, a relayhaving an operating winding connected across said conductors at theother end thereof, said relay having contacts which are picked up when apredetermined minimum value of energy is supplied to said winding oversaid conductors, means connected at one end of said conductors forrecurrently changing the value of energy supplied from said source tosaid winding between a first value which is greater than saidpredetermined value and a second value which is greater than said firstvalue, and means connected at the other end of said pair of conductorsresponsive to recurrent changes in the value of energy supplied fromsaid source to said conductor, said changes being at least as great asthe difference between said first and said second values.

3. In combination, a section of railway track, a track battery connectedacross the rails of said section at one end thereof, a track relayhaving a winding connected across the rails of said section at the otherend thereof, said track relay having a contact which is picked up onlywhen energy of a predetermined value is supplied to the relay windingover the rails of said section from said track battery, a first meansfor at times recurrently changing the value of energy supplied to saidrelay winding between a first value which is greater than saidpredetermined value and a second value which is greater than said firstvalue, and second means for detecting the recurrent changes in value ofthe energy supplied to said winding, said first and said second meansbeing connected to said track sections at opposite ends thereof.

4. In combination, a section of railway track, a track battery connectedacross the rails of said section at one end thereof, a track relayhaving a winding connected across the rails of said section at the otherend thereof, said track relay having a contact which is picked up onlywhen energy of a predetermined value is supplied to the relay Windingover the rails of said section from said track battery, a first meansfor at times recurrently changing the value of energy supplied to saidrelay winding between a first value which is greater than saidpredetermined value and a second value which is greater than said firstvalue, and second means for detecting the recurrent changes in the valueof the energy supplied to said relay winding, comprising a transformerhaving a first winding interposed in the connection between said batteryand said relay winding including the rails of said section, a codefollowing relay having a winding connected to a second winding of saidtransformer, and code detecting means .for detecting the recurrentoperation of said code following relay, said first and said second meansbeing connected to said track circuit at opposite ends thereof.

5. In combination, a section of railway track, a track battery connectedacross the rails of said section at one end thereof, a track relayhaving a winding connected across the rails of said section at the otherend thereof, said track relay having a contact which is picked up onlywhen energy of a predetermined value is supplied to the relay windingover the rails of said section from said track battery, a recurrentlyoperated contact, a resistor, means including said recurrently operatedcontact for at times shunting said resistor across said track relaywinding, said resistor being selected so that the value of energysupplied to the winding of said track relay when said resistor isshunted across the winding is at least as great as said predeterminedvalue, a transformer having a first and a second winding, said firstwinding being connected in series with said track battery across therails of the section, a code following relay having a winding connectedacross said second winding of said transformer, and means for detectingthe code following operation of said code following relay, the partsbeing proportioned so that the energy induced in the second winding ofsaid transformer by the current changes in the first winding of saidtransformer when said track relay winding is recurrently shunted by saidresistor is suflicient to operate the code following relay.

6. In combination, a section of railway track, a track battery connectedacross the rails of said section at one end thereof, a track relayhaving a winding connected across the rails of said section at the otherend thereof, said track relay having a contact which is picked up onlywhen energy of a predetermined value is supplied to the relay windingover the rails of said section from said track battery, coding meanslocated at each end of said section for recurrently changing the valueof energy supplied to said relay winding between a first value which isgreater than said predetermined value and a second value which isgreater than said first value, selecting means for energizing saidcoding means at one end or the other end of said section, and meanslocated at each end of said section for detecting recurrent changes inthe energy supplied to said section between said first value and saidsecond value.

7. In combination, a section of railway track, a track battery connectedacross the rails of said section at one end thereof, a track relayhaving a winding connected across the rails of said section at the otherend thereof, said track relay having a contact which is picked up onlywhen energy of a predetermined value is supplied to the relay windingover the rails of said section from said track battery, coding meanslocated at each end of said section for recurrently changing the valueof energy supplied to said relay winding between a first value which isgreater than said predetermined value and a second value which isgreater than said first value, selecting means for energizing saidcoding means at one end or the other end of said section, means locatedat each end of said section for detecting recurrent changes in theenergy supplied to said section, comprising a transformer having a firstand a second winding, said first Winding being connected to the trackrails of the section so that the energy flowing in said track rails alsoflows through said. first winding, a code following relay having awinding connected to said second winding of the transformer, and codedetecting means for detecting the recurrent operation of said codefollowing relay, the parts being proportioned so that the code followingrelay will operate in response to the changes of energy in said sectionbetween said first and said second values.

8. In combination, a section of railway track, a track battery connectedacross the rails of said section at one end thereof, a track relayhaving a winding connected across the rails of said section at the otherend thereof, said track relay having a contact which is picked up onlywhen energy of a predetermined value is supplied to the relay windingover the rails of said section from said track battery, a first codefollowing relay, a first transformer having a first winding connected inseries with said track relay winding and having a second windingconnected at times to a winding of said first code following relay andadapted to be recurrently connected at other times to a first source ofdirect current energy, a second code following relay, a secondtransformer having a first winding connected in series with said trackbattery and having a second winding connected at times to a winding ofsaid second code following relay, and adapted to be recurrentlyconnected at other times to a second source of direct current energy,code detecting means associated with each of said code following relaysand governed thereby to detect the code following operation of theassociated relay, selecting means for selectively recurrently connectingone or the other of said second windings of said transformers to theassociated source of direct current energy, and control circuitsgoverned by the contact of said track relay and by said code detectingmeans.

9. In combination, a pair of conductors extending between two spacedpoints, a source of direct current, a winding, said source and saidwinding connected in series across said conductors at a first one ofsaid points, a direct current electro-responsive device connected acrosssaid conductors at a second one of said points and effectively energizedin response to a predetermined minimum value of direct current suppliedby said source to said conductors, coding means having a contactrecurrently operated between two positions, means including impedanceelements and said contact connected across said conductors at saidsecond point for recurrently changing the value of the direct currentflowing from said source to said winding through said conductors, saiddirect current being changed between a first value which is at least asgreat as said predetermined value and a second value greater than saidfirst value, and another electro-responsive device coupled to saidwinding and responsive to changes of direct current flowing through saidwinding when said changes are recurrent and are at least as great as thedifference between said first and said second values.

10. In combination, a track section provided with a direct current trackcircuit including a direct current source connected across the rails atone end of the section and a direct current relay connected across therails at the other end of the section, said relay being effectivelyenergized in response to a predetermined minimum value of currentsupplied by said source to said track circuit, a Winding interposed inthe connection of said source to said track circuit, coding means havinga contact recurrently operated between two positions, means includingsaid contact and impedance elements connected across the rails at saidother end of the section for recurrently changing the value of thedirect current flowing from said source through said winding to therails of said section, said change being between a first value which isat least as great as said predetermined value and a second value whichis greater than said first value, and an electroresponsive means coupledto said winding and responsive to changes of direct current flowingthrough said winding when said changes are recurrent and are at least asgreat as the difference between said first and said second values.

11. In combination, a pair of conductors, a source of direct currentenergy connected across said conductors at one end thereof, anelectro-responsive device connected across said conductors at the otherend thereof, said electro-responsive device being responsive to apredetermined minimum value of energy supplied thereto over saidconductors, means located at one end of said pair of conductors forrecurrently changing the value of energy supplied from said source tosaid electro-responsive device between a first value which is greaterthan said predetermined minimum value and a second value which isgreater than said first value, and means located at the other end ofsaid pair of conductors responsive to changes of energy supplied fromsaid source to said conductors, when said changes are recurrent and areat least as great as the difference between said first and said secondvalues, said last named means comprising a transformer having a firstand a second winding, said first winding being connected in series withone of said pair of conductors, a code following relay having anoperating winding connected to the second winding of said transformer,and having contacts which are operated recurrently in response to saidrecurrent changes, and code detecting means responsive to recurrentoperation of the contacts of said code following relay.

References Cited in the file of this patent UNITED STATES PATENTS Clarket al. Dec. 3, Baughman July 15, Pelikan Feb. 10, Dicke Sept. 26,Willing Nov. 8, Sorensen Nov. 26, Baughrnan May 22, Coley Aug. 7, BarkerJune 17,

